In an electronic device such as a semiconductor element, semiconductor parts such as transistors, resistors and the like are arranged on a substrate. Those parts must be electrically isolated from each other, and hence among them it is necessary to form an area separating them. This area is referred to as an “isolation area”. Hitherto, the isolation area has been generally provided by forming an insulating film selectively on the surface of the semiconductor substrate.
Meanwhile, recently in the field of electronic device technology, the density and the integration degree have been more and more increased. According as the density and the integration degree are becoming higher, it is getting more difficult to form an isolation structure having fineness corresponding to the required integration degree. It is, therefore, desired to provide a new isolation structure satisfying the required fineness. As one of the isolation structures capable of satisfying the requirement, a trench isolation structure is proposed. The trench isolation structure is fabricated by forming fine trenches on a semiconductor substrate and then by filling the trenches with insulating material so as to electrically separate the part positioned on one side from that on the other side of each trench. The structure thus electrically separating the parts can reduce the isolation area, as compared with the conventional insulating film, and accordingly is effective in achieving the integration degree required in these days.
For fabricating the trench isolation structure, a CVD (chemical vapor deposition) method or a high density plasma CVD method, for example, can be used (see, for example, Patent document 1). However, if the trenches having fineness required in these days, for example, the trenches of 30 nm or less are filled in according to those methods, some voids are often contained in the filled trenches. These structural defects are liable to deteriorate the physical strength of the substrate and/or to impair the isolation characteristics.
In order to favorably fill the trenches in, it is proposed (for example, in Patent documents 1 and 2) that a polysilazane composition be applied on the substrate and then converted into silicon dioxide to fill the trenches with. When the polysilazane is converted into silicon dioxide, its volume shrinkage is little enough to avoid cracks formed by the volume shrinkage.
However, in the above method, it is necessary for the coating layer of the composition to undergo an oxidation process so as to be used as a silicon dioxide film. The oxidation process needs oxygen atom-containing ions, radicals or molecules, but these species are supplied only from the surface of the applied composition layer. Accordingly, the thicker the polysilazane layer is, the less the oxygen atoms migrate in the layer from the surface to the interface with the substrate. As a result, nitrogen atoms originally contained in polysilazane compounds in the composition are liable to remain near the interface with the substrate (see, Patent document 3). If the oxidation is thus so insufficient that nitrogen atoms remain near the interface, fixed charges may be generated and strong flat band shift due to the fixed charges may give unfavorable effects to the functions of semiconductor parts such as transistors formed from the silicon dioxide film. Further, Si—NH moieties remain in the film may be gradually oxidized as time elapses, and consequently the characteristics of semiconductor parts may be changed to impair the performance stability thereof.    [Patent document 1] Japanese Patent No. 3178412 (paragraph: 0005 to 0016)    [Patent document 2] Japanese Patent Laid-Open No. 2001-308090    [Patent document 3] Japanese Patent Laid-Open No. 2007-36267    [Patent document 4] Japanese Patent Laid-Open No. 08 (1996)-125021    [Patent document 5] Japanese Patent Laid-Open No. 01 (1989)-24852